Electronic devices and circuits, in general, consist of an active layers or components of light emitting cells, charge transfer layers, inks, bus bars, etc., disposed between a substrate/back sheet (hereinafter interchangeably used) and a cover/front sheet (hereinafter interchangeably used), and the substrate and cover sheets are adhered together with an encapsulated adhesive or a laminated adhesive film that enhances light transmission and optical effects. One or both of the substrate and cover are made of optical transparent materials. The active layers and/or components in electronic devices are sometimes susceptible to degradation by moisture and oxygen. A laminating adhesive film can restrict the transmission of oxygen and water vapor, and protects the active layers from degradation.
The use of a laminating adhesive film and/or an encapsulant (hereinafter interchangeably used) in an electronic device improves manufacturing efficiency over a liquid adhesive. However, drawbacks associated with films include poor substrate wet-out and poor void filling during assembly process due to the viscoelasticity of the film. This problem is exacerbated for substrates that contain components such as, electrodes, bus bars, ink steps, integrated circuits, and the like, due to their irregular surfaces, increasing the likelihood of void formation. WO2009148722 and WO2011062932 disclose the use of high (typically greater than 300,000g/mol) weight average molecular weight (Mw) polyisobutylene-based adhesives. Such adhesives have high viscosity, and thus, are susceptible to voids or air bubbles in organic electronic devices. In order to obtain better wet out, more ink filling, less air bubbles/ voids, the uncured adhesive film is applied onto the substrate under hot lamination. However, many active organic and electronic components are sensitive to heat above 60° C., and prolonged exposure to heat results in detrimental effects on the polymer and components. JP2012057065 discloses non-curable adhesive with pressure sensitive adhesive properties. In order to properly wet-out the substrates and to minimize void formations, the viscosity of the adhesive film is kept below 1,000,000 cps or below 200,000 g/mol viscosity average molecular weight (Mv) at 120° C.; however, the uncured thermal plastic adhesive exhibits cold flow under strain during the lifetime of the device.
CN 103820042 discloses the use of SIS and SBS block copolymer to make a heat-curable hot melt adhesive for Thermal-melt Optical Clear Adhesive (TOCA). However, it is well known in the art that the unsaturated C═C functional bonds in the soft block of SIS and SBS block copolymer readily oxidizes in air under UV light or at elevated temperature, and the adhesive film will turn yellow or brown over time.
Optically clear adhesive films (LOCA, OCA or UV OCA) pose another reworkability issue to the assembly process for electronic devices, such as display panels, glass plates, touch panels, diffusers, rigid compensators, heaters, and flexible polarizers. WO201429062 describes the challenge of the reworking process when electronic devices are formed with UV curable liquid optically clear adhesive (LOCA). During the reworking process, the cover sheet and LCD panel are separated either by hand or wire cutting, the LOCA residue on both cover sheet and substrate is then removed and cleaned with an organic solvent. Organic solvent is typically used to remove the adhesive residue, and this presents environmental, health and safety concerns. If the reworking process is carried out after UV cure, the adhesive film is usually broken into many small solid pieces, and removing all of these pieces can be time-consuming.
WO2013173976A1 discloses another challenge for optical devices, which includes appearance of dark spots and patches on LCD panels, also known as “Mura.” Mura describes a display effect of low-contrast, irregular pattern or region, which is usually caused by uneven screen uniformity or a localized stress. One source of Mura is from the optically clear adhesive. Any kind of stress, even at low levels, inside the display can cause Mura and it is not a repairable defect. The Mura can be minimized if the optically clear adhesive films have ˜0% shrinkage after cure and form soft adhesive films.
U.S. Pat. Nos. 5,559,165 and 6,448,303 disclose hot melt pressure sensitive adhesives that function as hydrogels. They are soft (Shore OO<30) and leaves no oil residue upon removal from a hard substrate. Other similar hydrogel-like soft adhesives were obtained by crosslinking or curing after cooling, as is the case with an electron beam curable acrylic described in EP175562. Similarly, U.S. Pat. No. 5,262,468 describes the use of high viscosity rubbers (40,000 cp at 25wt % in toluene) to obtain gelatinous thermoplastic compositions, but these compositions generally lack adhesive grab and virtually no adhesion exists.
There is a need in the art for a curable optically clear pressure sensitive adhesive that can be used as laminating adhesive or encapsulant for electronic devices and allow for good substrate wet-out and reworkability. The current invention fulfills this need.